Decline in wintertime air-mass transition frequencies in the USA

Air-mass transitions are an important component of atmospheric circulation, but are difficult to objectively identify and analyze in the context of decadal-scale climate change. Spatial synoptic classification (SSC) uses surface weather observations of daily dew point temperature range, sea-level pressure range, and wind shift to identify days with a distinct change from one air mass to another ('transition days'). Thus, the SSC offers a unique opportunity to examine the climatology of synoptic-scale transition events. The frequency of the transition type has significantly declined across United States over the second half of the twentieth century, potentially revealing a climatic shift toward fewer abrupt air-mass changes, but the underlying causes have not been well-documented to date. We link the decline in winter transition frequency to decreasing intra-daily moisture and pressure variability by examining the largest within-day dew point temperature and sea-level pressure changes. Fewer transitions are being observed in the continental United States because of increasing daily minimum dew point temperatures in the upper Midwest and smaller pressure changes over 24 h periods in the western states. The frequency and intensity of air-mass transitions are closely tied to other components of the climate system; therefore, examination of transition trends serves as a novel framework for exploring climatic change.